The ball grid array package is the more advanced electronic package type. FIG. 1 shows a chip device of cavity down type. A metal plate is disposed on the back of a substrate a to enhance the heat dissipating effect. A concavity al forms on the front surface of the substrate a. A chip c is attached in the concavity al by adhesives such as conducting epoxy or silver epoxy. The front surface of the substrate a has bonding areas a2 adjoining to the concavity a1. Metal leads 1 are formed by wire-bonding to connect the bonding areas a2 to bonding pads c1 of the chip c. An encapsulation layer d is formed to sheathe the chip c and the bonding areas a2 of the substrate a. Solder balls a3 on the substrate a are soldered to another device b such as a motherboard.
As the packing density of IC devices increases, their circuit functions become more complex. Using the back surface of the substrate a for heat dissipation of the chip c is not enough. Other means must be adopted to enhance the heat dissipating effect.